Process control systems are widely used in factories and/or process plants in which products are manufactured or processes are controlled (e.g., chemical manufacturing, power plant control, oil refinery, etc.) to produce a product of some sort. Process control systems are also used in the harvesting of natural resources such as, for example, oil and gas drilling and handling processes, etc. Virtually any manufacturing process, resource harvesting process, etc. can be automated through the application of one or more process control systems.
The manner in which process control systems are implemented has evolved over the years. Older generations of process control systems were typically implemented using dedicated, centralized hardware. However, modern process control systems are typically implemented using a highly distributed network of workstations, intelligent controllers, smart field devices, and the like, some or all of which may perform a portion of an overall process control strategy or scheme. In particular, most modern process control systems include smart field devices and other process control components that are communicatively coupled to each other and/or to one or more process controllers via one or more digital or combined digital and analog data busses. Of course, many of these modern process control systems may also include non-smart field devices such as, for example, 4-20 milliamp (MA) devices, 0-10 volts direct current (VDC) devices, etc., which are typically directly coupled to controllers, as opposed to using a shared digital data bus or the like.
More particularly, modern process control systems, such as distributed or scalable process control systems, typically include one or more process controllers communicatively coupled to each other, to at least one host or operator workstation and to one or more field devices via analog, digital or combined analog/digital buses. The field devices, which may be, for example, valves, valve positioners, switches and transmitters (e.g., temperature, pressure and flow rate sensors), perform functions within the process such as opening or closing valves and measuring process parameters. The process controller receives signals indicative of process measurements made by the field devices and/or other information pertaining to the field devices, and uses this information to implement one or more control routines to generate control signals which are sent over the buses to the field devices to control the operation of the process. Information from the field devices and the controller is typically made available to one or more applications executed by the operator workstation to enable an operator to perform any desired function with respect to the process, such as viewing the current state of the process, modifying the operation of the process, etc.
Some process control systems, such as the DeltaV™ system sold by Emerson Process Management, use function blocks or groups of function blocks, referred to as modules, located in the process controller or in different field devices or input/output (I/O) controller devices to perform control operations. In these cases, the process controller or other device is capable of including and executing one or more function blocks or modules, each of which receives inputs from and/or provides outputs to other function blocks (either within the same device or within different devices), and performs some process control operation, such as measuring or detecting a process parameter, controlling a device, or performing a control operation, such as implementing a proportional-derivative-integral (PID) control routine. The different function blocks and modules within a process control system are generally configured to communicate with each other (e.g., over a bus) to form one or more process control loops.
In many cases, process controllers are programmed to execute a different algorithm, sub-routine or control loop (which are all control routines) for each of a number of different loops defined for, or contained within a process, such as flow control loops, temperature control loops, pressure control loops, etc. Generally speaking, each such control loop includes one or more input blocks, such as an analog input (AI) function block, a control block, such as a proportional-integral-derivative (PID) or a fuzzy logic control function block, and an output block, such as an analog output (AO) function block. Control routines, and the function blocks that implement such routines, have been configured in accordance with a number of control techniques including, for example, PID control, fuzzy logic control, and model-based control techniques such as a Smith predictor or model predictive control (MPC).
This increased amount of controller functionality results in increased levels of data transfer that must occur between different devices within a process control system to support the controller functionality. Thus, one particularly important aspect of modern process control system design involves the manner in which field devices are communicatively coupled to each other, to the process controllers and to other systems or devices within a process control system or a process plant. In general, the various communication channels, links and paths that enable the field devices to function within the process control system are commonly collectively referred to as an input/output (I/O) communication network.
The communication network topology and physical connections or paths used to implement an I/O communication network can have a substantial impact on the robustness or the integrity of field device communications, particularly when the I/O communication network is subjected to environmental factors or conditions associated with the process control system. For example, many industrial control applications often subject field devices and their associated I/O communication networks to harsh physical environments (e.g., high, low or highly variable ambient temperatures, vibrations, corrosive gases or liquids, etc.), difficult electrical environments (e.g., high noise environments, poor power quality, and transient voltages), etc. As a result, numerous different types of I/O communication networks and communication protocols have been developed to be used to provide communications on those networks.
More particularly, to support the execution of the control routines in a distributed process control system, a typical industrial or process plant has a centralized control room that is communicatively connected with one or more of the distributed process controllers and process I/O subsystems which, in turn, are connected to the one or more field devices that perform control activities within the plant, such as measuring process variables or performing physical actions in the plant (e.g., opening or closing a valve). Traditionally, analog field devices have been connected to the controller by two-wire or four-wire current loops for both signal transmission and the supply of power. An analog field device that transmits a signal to the control room (e.g., a sensor or a transmitter) modulates the current running through the current loop, such that the current is proportional to the sensed process variable. On the other hand, analog field devices that perform an action under control of the control room are controlled by the magnitude of the current through the loop.
More recently however, process control communication systems have been developed that superimpose digital data on the current loop used to transmit the analog signals. For example, the Highway Addressable Remote Transducer (HART®) protocol uses the loop current magnitude to send and receive analog signals, but also superimposes a digital carrier signal on the current loop signal to enable two-way field communication with smart field instruments. Still further, other protocols have been developed that provide all digital communications on a bus associated with an I/O communication network. For example, the FOUNDATION® Fieldbus protocol, which is generally referred to as the Fieldbus protocol, provides all digital communications on a bus associated with an all-digital I/O communication network. The Fieldbus protocol actually includes two sub-protocols, including the H1 protocol which supports data transfers at a rate up to 31.25 kilobits per second while powering field devices coupled to the network, and the H2 protocol which supports data transfers at a rate up to 2.5 megabits per second but without providing power to the field devices via the bus. With these types of communication protocols, smart field devices, which are typically all digital in nature, support a number of maintenance modes and enhanced functions not provided by older control systems. However, these digital based communication protocols also typically require a bus controller device, sometimes referred to as a link controller device, to assure proper communications on the bus, to interface to external devices, such as process controllers and user interface devices that are not attached to the I/O communication network, etc.
As noted above, some of the I/O communication networks and the protocols associated with these networks have been developed to provide power to the field devices connected to the network bus in addition to communicating digital and/or analog signals on the network bus. Providing power on the network bus (referred to herein as bus power) enables the I/O communication network itself to power the field devices and other devices connected to the I/O communication network, thereby eliminating the need to provide a separate power source for each field device, controller, etc. connected to the I/O communication network. This feature is very useful in process control systems that are implemented outdoors, in harsh environments, or in remote or not easily accessible locations. However, the bus power feature is also very useful in enclosed plants and other more traditional locations, as it reduces the cabling and wiring needed to provide separate power signals to each of the field devices within a process control system.
Typically, I/O communication networks that provide bus power include a separate power module or power supply device that is connected to the bus to place the appropriate power signal onto the bus to be used to power the other devices connected to the bus. In some cases, such as in the Fieldbus H1 protocol, the power supply may be redundant in nature and may be isolated from the bus by an impedance network that prevents the power supply from interfering with the flow of digital signals on the network bus. Thus, in many instances, the configuration of an I/O communication network that provides bus power requires that separate power supply devices be connected to the bus, in addition to the bus controller and the field devices connected to the bus, to be able to provide power on the bus. These systems may also require additional devices disposed between the power supply devices and the bus to isolate the power supplies from the digital communications on the network bus. These requirements lead to additional hardware and wiring being needed for the I/O communication network, require additional space in the cabinets which house the hardware for the I/O communication network, and require additional configuration and wiring activities when setting up and configuring the I/O communication network. Moreover, the additional set up and configuration procedures, which generally entail setting up and wiring hardware together to create the I/O communication network, lead to more errors and potential problems in the implementation and running of a particular I/O communication network.